Take only photographs, leave only footprints. Its the mantra for things, items on pubic land. If you take it, you are denying someone else of experiencing of seeing the same thing you did when you saw it. If something is on public land, it belongs to all the citizens. It doesn't matter that there are lots of them or hardly anyone goes there.
Of course there are always exceptions, like certain plants and if hunting is allowed, but for human made things, it is not yours and you should not take it, unless it is modern litter.
Steve